There are numerous methods for manufacturing gear wheels. In soft preliminary machining one distinguishes hobbing, gear shaping, generating planing, and power skiving. Hobbing and skiving are so-called continuous methods, as explained in greater detail hereafter.
In cutting gear wheels, one differentiates between the intermittent indexing method or single indexing method and the continuous method, sometimes also designated as the continuous indexing method or face hobbing.
In the continuous method, for example, a tool having corresponding cutters is used to cut the flanks of a work piece. The work piece is finish cut continuously in a chuck, i.e., in an uninterrupted method. The continuous method is based on complex, coupled movement sequences, in which the tool and the work piece to be machined execute a continuous indexing movement relative to one another. The indexing movement results from the coordinated or coupled driving, respectively, of multiple axis drives of a corresponding machine.
In the single indexing method, one tooth gap is machined, then, for example, a relative movement of the tool and a so-called indexing movement (indexing rotation) occur, during which the work piece rotates relative to the tool before the next tooth gap is then machined. A gear wheel is thus manufactured step-by-step.
The gear shaping method mentioned at the beginning can be described or represented by a cylinder wheel gear, since the intersection angle (also called axis intersection angle) between the rotational axis R1 of the shaping tool 1 and the rotational axis R2 of the work piece 2 is zero degrees, as schematically shown in FIG. 1. The two rotational axes R1 and R2 extend in parallel if the axis intersection angle is zero degrees. The work piece 2 and the shaping tool 1 continuously rotate around their rotational axes R2 or R1, respectively. The shaping tool 1 performs a stroke movement in addition to the rotational movement, which is indicated in FIG. 1 by the double arrow Shx, and removes chips from the work piece 2 during this stroke movement.
Some time ago, a method which is designated as skiving was taken up again. The fundamentals are approximately 100 years old. A first patent application having the number DE 243514 on this subject dates back to the year 1912. After the original considerations and studies of the initial years, skiving was no longer seriously pursued further. This is because complex processes, which were partially empirical, were necessary to find a suitable tool geometry for the skiving method.
About in the middle of the nineteen-eighties, skiving was taken up again. The principle of skiving could first be implemented in a productive, reproducible, and robust method with current simulation methods and modern CNC controllers of the machines. In addition, the high wear resistance of current tool materials, the enormously high static and dynamic stiffness, and the high quality of the synchronized running of the modern machines were significant.
As shown in FIG. 2A, in skiving, an axis intersection angle Σ between the rotational axis R1 of the skiving tool 10 (also designated as the skiving wheel) and the rotational axis R2 of the work piece 20 is predefined, which is not equal to zero. The resulting relative movement between the skiving tool 10 and the work piece 20 is a helical movement, which can be decomposed into a rotary component (rotational component) and an advance component (translational component). A rolling helical gearing can be considered to be a drive-technology analogue, the rotary component corresponding to the rolling and the advance component corresponding to the sliding of the flanks. The greater the absolute valve of the axis intersection angle Σ, the more the translational movement component required for the machining of the work piece 20 increases. This is because it causes a movement component of the blades of the skiving tool 10 in the direction of the tooth flanks of the work piece 20. In skiving, the sliding component of the meshing relative movement of the engaged gear wheels of the equivalent helical gearing is utilized to execute the cutting movement. In skiving, only a slow axial feed sax (also called axial feed) parallel to the rotational axis R2 of the work piece 20 is required and therefore the so-called shaping movement, which is typical for gear shaping, is omitted. Also, no return stroke movement therefore occurs in skiving.
The cutting velocity in skiving is directly influenced by the speed of the skiving tool 10 or the work piece 20 and by the employed axis intersection angle Σ of the rotational axes R1 and R2. The axis intersection angle Σ, and therefore the sliding component, are to be selected so that an optimum cutting velocity is achieved for the machining of the material at a given speed.
The movement sequences and further details of a previously known skiving method can be inferred from the above-mentioned schematic illustration in FIG. 2A. FIG. 2A shows the skiving of external gear teeth on a cylindrical work piece 20. The work piece 20 and the tool 10 (a cylindrical skiving tool 10 here) rotate in opposite directions, as can be seen in FIG. 2A, e.g., on the basis of the angular velocities ω1 and ω2.
In addition, there are further relative movements. The above-mentioned axial feed sax is required to be able to machine the entire gear teeth width of the work piece 20 using the tool 10. The axial feed causes a displacement of the tool 10 in relation to the work piece 20 in the parallel direction to the rotational axis R2 of the work piece 20. The direction of this movement of the tool 10 is identified in FIG. 2A by sax. If helical gear teeth are desired on the work piece 20 (i.e., β2≠0), a differential feed sD is overlaid on the axial feed sax, which, as indicated in FIG. 2A, corresponds to an additional rotation of the work piece 20 around its rotational axis R2. The differential feed sD and the axial feed sax are adapted to one another at the design point such that the resulting feed of the tool 10 in relation to the work piece 20 occurs in the direction of the tooth gap to be generated. In addition, a radial feed srad can be used to influence the crowning of the gear teeth of the work piece 20.
In skiving, the vector of the cutting velocity {right arrow over (ν)}c results substantially as the difference of the two velocity vectors {right arrow over (ν)}1 and {right arrow over (ν)}2 of the rotational axes R1, R2 of tool 10 and work piece 20, which are inclined to one another by the axis intersection angle Σ. {right arrow over (ν)}1 is the velocity vector at the periphery of the tool 10 and {right arrow over (ν)}2 is the velocity vector at the periphery of the work piece 20. The cutting velocity νc of the skiving process can be changed by the axis intersection angle Σ and the speed in the equivalent helical gearing. The axial feed sax, which is relatively slow as already mentioned, only has a small influence on the cutting velocity νc, in the skiving method, which can be neglected. The axial feed sax is therefore not taken into consideration in the vector diagram having the vectors {right arrow over (ν)}1, {right arrow over (ν)}2, and {right arrow over (ν)}c in FIG. 2A.
FIG. 2B shows the skiving of external gear teeth of a work piece 20 using a conical skiving tool 10. FIG. 2B again shows the axis intersection angle Σ, the vector of the cutting velocity {right arrow over (ν)}c, the velocity vectors {right arrow over (ν)}1 on the periphery of the tool 10 and {right arrow over (ν)}2 on the periphery of the work piece 20, and the helix angle β1 of the tool 10 and the helix angle β2 of the work piece 20. The helix angle β2 is not equal to zero here. The tooth head of the tool 10 is identified in FIG. 2B by the reference sign 4. The tooth face is identified in FIG. 2B by the reference sign 5. The two rotational axes R1 and R2 do not intersect, but rather are arranged skewed to one another. With a conical skiving tool 10, the design point AP is typically selected on the common perpendicular of the two rotational axes R1 and R2, since tilting of the skiving tool 10 is not necessary to provide clearance angles. The design point AP is coincident here with the so-called contact point BP. The pitch circles of the equivalent helical rolling gearing touch at this design point AP.
A tool 10 is used in skiving, which comprises at least one geometrically defined flank cutting edge. The flank cutting edge(s) are not shown in FIG. 2A and FIG. 2B. The shape and arrangement of the flank cutting edges are among the aspects which must be taken into consideration in practice in a concrete design.
In addition, the tool itself has great significance in skiving. The skiving tool 10 has the shape of a straight-toothed spur gear in the example shown in FIG. 2A. The outer contour of the main body in FIG. 2A is cylindrical. However, it can also be cone-shaped (also called conical), as shown in FIG. 2B. Since the gear tooth or gear teeth of the skiving tool 10 engage over the entire length of the flank cutting edge, each gear tooth of the tool 10 requires a sufficient clearance angle at the flank cutting edge.
It is known that a so-called semi-completing approach can be followed in skiving. An approach is designated as a semi-completing method, in which both right and also left flanks of tooth gaps are machined in a first step, but only the geometries of the right or left flanks are finish machined. Then, in a second step, after the machine setting has been changed, one of the two flanks is reworked to obtain the desired gap width and tooth geometry. One reason for the application of a semi-completing method is that the flanks can be designed more freely, i.e., so-called flank modifications are more easily possible than in the completing method. In addition, the tooth thickness can also be corrected via the semi-completing approach, in that the gap width is changed by a simple pivot of the work wheel.
The semi-completing method is originally known for bevel gears from grinding in the single indexing method of gear teeth which are premilled in the Zyklo-Palloid® method.
Several studies of previous skiving methods have shown that, depending on the design of the skiving tools 10, significant wear of the skiving tool 10 can occur. The statement also applies for the application of the semi-completing approach to skiving. Therefore, solutions are sought which allow the wear of the skiving tools 10 to be reduced, or the service life of the skiving tools 10 to be improved, respectively. The skiving method becomes more cost-effective through reduced wear, since the production costs during the gear cutting of work pieces 20 are substantially influenced by the tool service lives.